This invention relates to controlling emissions of volatile sulfur compounds, and more particularly to an improved method and apparatus for removal of a volatile divalent sulfur compounds from a contaminated gas stream. The novel method and apparatus are particularly suited for removal of both volatile divalent sulfur compounds and volatile organic carbon compounds from gas streams.
Thermal and catalytic oxidation processes have commonly been used for removal of volatile organic compounds from effluent gas streams. For example, Matros U.S. Pat. Nos. 4,877,592 and 5,364,259 describe processes in which the volatile organic compounds are burned with oxygen contained in the gas stream. The heat of combustion is recovered by regenerative heat transfer and used to preheat the gas stream to combustion temperature. Thus, the method of the Matros patent is effective for preventing air pollution due to emissions of hydrocarbon gases and vapors. However, the Matros patent does not disclose any method for removing volatile divalent sulfur compounds from an effluent gas stream, or for treating a gas stream that contains both volatile organic compounds and volatile divalent sulfur compounds.
Various industrial processes generate gaseous effluent streams that contain volatile divalent sulfur compounds. For example, the Claus process is effective for economic recovery of sulfur from hydrogen sulfide and mercaptans, but nonetheless typically generates a waste stream containing substantial residual quantities of H.sub.2 S, COS, CS.sub.2, SO.sub.2 and mercaptans. Internal combustion engine and power generation exhaust gases also typically contain hydrocarbons and sulfur compounds in various state of oxidation. Certain chemical manufacturing processes also produce effluent gases that contain volatile divalent sulfur compounds, including volatile organic sulfur compounds, sometimes in admixture with volatile hydrocarbons.
Processes are known for elimination of the divalent sulfur compounds by catalytic oxidation. Thus, Dupin U.S. Pat. No. 4,937,058 discloses a catalyst for oxidizing sulfur-containing effluent gases from the Claus process. The catalyst is described as comprising metals selected from among Cu, Ag, Zn, Cd, Y, lanthanides, Cr, Mo, W, Mn, Fe, Co, Rh, Ir, Ni, Pd, Pt, Sn, and Bi on a support comprising TiO.sub.2, ZrO.sub.2, SiO.sub.2 --MgO, SiO.sub.2 --ZrO.sub.2, ZrO.sub.2 --TiO.sub.2 or their mixtures on a zeolite, with the proviso that the catalyst not contain spinel-type mixed oxides.
Soviet Patent 1,583,358 describes a process for deep oxidation of divalent sulfur compounds over a catalyst comprising 15-20% CuCr.sub.2 O.sub.4, 5-7% CuO and 10-12% Cr.sub.2 O.sub.3 on and Al.sub.2 O.sub.3 support.
Mulina et al., "CO Oxidation Over CuCr/Al.sub.2 O.sub.3 Catalysts in Presence of SO.sub.2, "React. Kinet. Catal. Lett.,Vol. 37, No. 1, 96-100 (1988) describes the catalytic oxidation of CO in a laboratory flow reactor charged with a catalyst containing Cr.sub.3 O.sub.3 and CuO in rations ranging from 0.75 to 1.91. Vanadium pentoxide was used as a promoter. The gas mixture entering the reaction zone contained 1 wt.% CO, 8 wt.% O.sub.2. The reference describes two types of oxide catalyst deactivation by SO.sub.2 : (1) fast, mainly at low temperatures due to the formation of surface sulfur compounds; and (2) slow, taking place at higher temperature over a prolonged period of time owing to formation of a sulfate phase. Some catalysts were regenerated by flowing air at 600.degree. C. and then tested in CO oxidation. Mulina et al. report that surface compounds are desorbed at 500.degree.-600.degree. C., whereas their decomposition takes place at temperatures significantly higher than 600.degree. C.
Kovalenko et al., "Dynamics of the Activity of Deep Oxidation Catalysts Under the Extreme, Conditions," (unpublished manuscript received January 1995) discusses the decrease in activity observed in deep oxidation of methane in flowing installations at initial CH.sub.4 and SO.sub.2 concentrations of 1.0% by volume and 0.5% by volume, respectively. An activity decrease caused by catalyst sample sulfation was observed. The inhibiting effect at 500.degree. C. was found to be stronger for alumino-manganese catalyst than for copper-chromium catalyst. The manuscript states "regeneration conditions of the manganese catalyst after sulfur poisoning with the complete activity reduction have been determined."
Cordonna et al. U.S. patent describes a sulfur tolerant Pt catalyst for the oxidation of volatile divalent sulfur compounds in flue gases from engines combusting fossil fuels.
Deeba et al. U.S. Pat. No. 5,145,825 describes a sulfur resistant catalyst for the oxidation of CO, hydrocarbons and SO.sub.x. The catalysts comprise Pt on a titania or zirconia support. The active catalyst phase can be included in a washcoat for a honeycomb carrier. The catalyst is particularly adapted for treatment of automobile exhaust gases, as well as exhaust gases from co-generation plants.
In the processes of Dupin, Cordonna, Mulina, Kovalenko, and Deeba, the reaction gas produced upon catalytic oxidation of sulfur compounds, hydrocarbons, CO, etc. is vented to the atmosphere and the combustion heat of the catalytic oxidation is apparently not recovered.
The Hydrocarbon Processing Process Gas Handbook 1990 describes a fluid bed system for oxidation of H.sub.2 S, organic sulfur compounds and chloro-organics in a combustion chamber directly fired with a fossil fuel. The contaminated gas stream is preheated by recuperative heat exchange with heat transferred from the combustion gas stream, and the latter stream is exhausted to the atmosphere.
Bayer et al. U.S. Pat. No. 5,262,131 describes a process in which volatile organic compounds are removed from an effluent stream by catalytic combustion, the contaminated gas stream entering the catalytic oxidation zone being preheated by recuperative heat exchange with the combustion gas exiting the oxidation zone.
In operation of the processes described above, sulfur oxides produced contained in the exhaust gases are released to the atmosphere. Processes are known for scrubbing of sulfur oxides from flue gases of power generation plants. Bramer U.S. Pat. No. 4,041,128 describes a system for removing SO.sub.2 from an internal combustion engine exhaust gas by passing it through a chamber located upstream of a catalytic converter. The chamber contains two compartments in series, the first compartment containing granular or powdered ferrous sulfide and a second compartment containing granular or powdered ferrous oxide. U.S. Pat. Nos. 3,443,886 and 3,429,656 disclose the use of calcium, sodium and silicon oxides as adsorbents for SO.sub.x, while U.S. Pat. No. 3,657,892 discloses the use of active carbon as an SO.sub.x adsorbent. However, there has been a continuing need for methods which remove volatile divalent sulfur compounds from effluent gases without creating excessive SO.sub.x emissions.